(1) Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device that discretely accumulates charges in a multilayer dielectric and a method for fabricating the same.
(2) Description of Related Art
In recent years, attention has been directed to nonvolatile semiconductor memory devices that discretely accumulate charges in multilayer dielectrics, because the devices provide high integration and high reliability.
By the way, it has been recognized that these nonvolatile semiconductor memory devices have the problem that the influence of ultraviolet light in a fabrication process of each nonvolatile semiconductor memory device disables control of a threshold voltage, leading to malfunction in writing and erasing operations. This problem will be described hereinafter in detail.
First, a nonvolatile semiconductor memory device according to a first background art will be described with reference to FIG. 5.
FIG. 5 is a cross-sectional view showing a typical nonvolatile semiconductor memory device that discretely accumulates charges in a multilayer dielectric.
As shown in FIG. 5, a multilayer dielectric 101 is formed, as a gate dielectric, by stacking, in bottom-to-top order, a lower oxide film 101a, a silicon nitride film 101b and an upper oxide film 101c on a silicon substrate 100. The multilayer dielectric 101 is used to accumulate charges. A polycrystalline silicon film 102 is formed as a gate electrode on the multilayer dielectric 101. Furthermore, diffusion regions 103 are formed in the surface of the silicon substrate 100 with the gate electrode interposed therebetween.
In a fabrication process of the nonvolatile semiconductor memory device shown in FIG. 5, elements are irradiated with ultraviolet light through various process steps after the formation of the gate electrode (for example, a lithography process step, a dry etching process step using plasma, a chemical vapor deposition (CVD) process step, and other process steps). When the gate electrode or the silicon substrate 100 is irradiated with ultraviolet light, electrons existing in the gate electrode or the silicon substrate 100 are excited. Among the excited electrons, electrons having obtained energy that can go beyond the energy barrier existing between the silicon substrate 100 and the multilayer dielectric 101 or between the gate electrode and the multilayer dielectric 101 are captured in the multilayer dielectric 101. In other words, the electrons captured in the multilayer dielectric 101 are accumulated in regions S1 of the multilayer dielectric 101 located in the vicinity of the ends thereof (hereinafter, referred to as “fixed charge accumulation region S1”). The light energy increases with decrease in wavelength. Therefore, visible light can be ignored.
In the nonvolatile semiconductor memory device shown in FIG. 5, writing and erasing operations are typically performed in the manner in which hot carriers are produced to locally capture charges in the multilayer dielectric 101 and extract the captured charges. More particularly, if electrons are captured in regions P1 of the multilayer dielectric 101 in which a writing or erasing operation is carried out (hereinafter, referred to as “writing/erasing regions P1”), i.e., regions of the multilayer dielectric 101 opposed to the vicinity of the interfaces between the diffusion regions 103 and a channel region existing between the diffusion regions 103, the threshold voltage is increased. On the other hand, if positive holes are captured in the writing/erasing regions P1, the threshold voltage is decreased. In this way, the writing and erasing operations are typically carried out in the writing/erasing regions P1.
However, when as shown in FIG. 5 the fixed charge accumulation regions S1 in which electrons captured in the interlayer dielectric 101 by the influence of ultraviolet light during various process steps are accumulated not only includes the writing/erasing regions P1 but also extends beyond the writing/erasing regions P1 into the surrounding regions of the multilayer dielectric 101, the threshold voltage cannot be changed to a desired value by the writing and erasing operations. The reason for this is that the value of the threshold voltage depends on the electrons captured during the various process steps. In summary, in the nonvolatile semiconductor memory device shown in FIG. 5, the threshold voltage cannot be controlled, and thus it is difficult to perform normal writing and erasing operations.
This problem is becoming more apparent with miniaturization of semiconductor memory devices. The reason for this is as follows: If device miniaturization is carried out according to a scaling law, it is possible to reduce in size the regions where the gate electrode overlaps with the diffusion regions 103, i.e., regions of the diffusion regions 103 located below the gate electrode. However, it is impossible to change in size also the fixed charge accumulation regions S1 of the multilayer dielectric 101 in which electrons are captured and accumulated by the influence of ultraviolet light.
It is needless to say that, even if in the above description electrons and positive holes are replaced with positive holes and electrons, respectively, the same description can be given.
On the other hand, a nonvolatile semiconductor memory device according to a second background art has been suggested to solve malfunction in writing and erasing operations due to the influence of ultraviolet light (see, for example, EP1313138 A2).
FIG. 6 is a cross-sectional view showing the structure of the nonvolatile semiconductor memory device according to the second background art.
In the nonvolatile semiconductor memory device 200A shown in FIG. 6, diffusion bit lines 201 are formed under parts of an oxide film 200, and writing and erasing operations are carried out by capturing charges in a multilayer film 202 obtained by successively stacking a silicon oxide film, a silicon nitride film and a silicon oxide film. According to the second background art, as shown in FIG. 6, a protective film (light shielding film) 203 is formed on memory elements to shield ultraviolet light, thereby performing normal writing and erasing operations.
However, the nonvolatile semiconductor memory device 200A according to the second background art has the following problems.
First, it is impossible to restrain charges from being captured in the multilayer dielectric by the influence of ultraviolet light in process steps until the formation of a light shielding film. More particularly, it is impossible to eliminate the influence of ultraviolet light produced in process steps from the formation of the multilayer dielectric to the formation of the light shielding film, because the light shielding film cannot be formed before the completion of memory cells.
Second, when contacts are to be formed everywhere in a memory cell array, short circuits may be caused through the contacts, because holes need be formed in the light shielding film to form the contacts. Thus, it is difficult to form contacts.
Third, since the light shielding film is formed in layers, the fabrication cost is increased.